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ok found the solution by myself... it was easier than I though.

It is needed to override the getCatImageLink method like the following

    public function getCatImageLink($name, $idCategory, $type = null)
    {
        if ($this->allow == 1 && $type
                && false        //add this
        ) {
            $uriPath = __PS_BASE_URI__ . 'c/' . $idCategory . '-' . $type . '/' . $name . '.jpg';
        } else {
            $uriPath = _THEME_CAT_DIR_ . $idCategory . ($type ? '-' . $type : '') . '.jpg';
        }

        return $this->protocol_content . Tools::getMediaServer($uriPath) . $uriPath;
    }